Case Study of a Road Embankment Failure Mitigated Using Deep Soil Mixing

This study investigates a slope failure in Northland, New Zealand mitigated using a combination of TurboJET deep soil mixing columns and drainage works. Ogles No. 3 site is situated in Allochthon residual soil which is prone to slope instability at low slope angles. The original analysis of the site attributed the failure of the embankment to transient groundwater flow and truck-induced dynamic loading. This study attempts to simplify the analysis by considering the reactivation of a slip surface as the primary cause for instability. The site is modelled in PLAXIS using the Mohr-Coulomb constitutive model, and the results correspond well with field observations. Regardless of the alternative interpretation of the failure mechanism, analysis of the DSM columns installed on site to mitigate slope failure indicates they are increasing the factor of safety and performing as intended. The installation of the columns increases the global factor of safety from approximately 1 to 1.3. At a reduced high groundwater level, groundwater cycling was found to have a small impact on total displacement at the road shoulder, and the DSM columns act to reduce this displacement.